Studies will be initiated to examine in greater detail the relationship between hormonal response and nuclear occupancy in an effort to determine whether the functional positive cooperativity noted in previous studies is generally applicable to other parameters of thyroid hormone action. An effort will be made to detail the nature of the biochemical events which follow nuclear binding and result in thyroid hormone action. The relationship between the molecular mechanisms described in the rat to hormonal response mechanisms in man will be analyzed. BIBLIOGRAPHIC REFERENCES: Surks, M.I., Koerner, D.H., and Oppenheimer, J.H.: In vitro binding of L-triiodothyronine to receptors in rat liver nuclei: Kinetics of binding, extraction properties, and lack of requirement for cytosol proteins. J. Clin. Invest. 55:50 1975. Stock, J.M., Ghatak, N.R., and Oppenheimer, J.H.: Unsuspected meningioma in a patient with pituitary gigantism. Case Report with autopsy findings. Metabolism 24:767, 1975.